Ophthalmologists and optometrists are sometimes called upon to evaluate the functional status of the visual system in terms of what activities an individual may engage in, in addition to diagnosing the presence of pathology. Requests for this kind of diagnosis may come from different sources such as government (disability payments), employers (meeting standards for employment) or state departments of motor vehicles (driving status). These types of evaluations may prove a difficult task since there is often a mismatch between sensory loss and the ability to function in the world--i.e., some individuals with dramatic sensory loss seem to function quite well while others, with only minor sensory loss, report great difficulty in visually guided activities. This difficulty may be exacerbated in the older population, where there is a marked increase in the range of functional disabilities, and often no clear distinction between aging and early onset of disease. Therefore, tests of functional vision are needed so evaluations can be made fairly, rather than based on age alone or on sensory tests alone which tend to be unrelated to behavioral measures.
Many different kinds of perimeters are available for assessing the integrity of the clinical visual field. Existing automated perimeters are used to screen for the presence of pathology (i.e., glaucoma or retinal damage), and may also be used to provide full threshold determination for many points throughout the visual field. These perimeters may employ projection techniques (for example, the Octopus Perimeter 500EZ by Interzeag Inc., Northboro, Mass. 01532) or use LED stimuli as targets, and can vary the backgrounds luminance, stimulus size and the exposure time of the targets. The software provided with these perimeters allows brief screening tests, a full threshold test, customized testing, electronic fixation monitoring with video cameras, an indication of short-term fluctuations, a comparison of thresholds obtained with normal values corrected for age, defect analysis, and tracking for visual field indices over time.
In comparison to these clinical visual fields, the useful field of view (UFOV) has been defined as the spatial area or visual field extent that is needed for a specific visual task. Unlike the traditional field, the UFOV is measured binocularly, employs suprathreshold rather than threshold targets, and both the target and background are complex. Although age-related deficits occur for both the clinical measures of the visual field, as obtained with standard perimetry and UFOV assessments, agerelated declines tend to be much larger for the UFOV measurements.
In addition to screening for pathology, standard perimetry has also been employed as a screening device for various behavioral measures such as driving. All research in this area has failed to find a relationship between the status of the visual field and difficulty in driving (as assessed by accident rate) except in a very small percentage of drivers with severe binocular field loss. Additionally, clinical visual field measures have not been found to be predictive of reported difficulties on visual tasks relating to visual search, mobility and speed of visual processing (as assessed by survey responses). Measures of the UFOV, however, are predictive of such reported difficulties. Thus, standard perimetric techniques underestimate the severity of many adults functional loss in the visual field. While older adults typically show some slight sensitivity losses throughout the visual field (relative to young adults), assessment of the UFOV can reveal up to a three-fold reduction in the field. Thus assessment of the UFOV could be used to delineate the perceptual functions necessary for the performance of routine activities dependent on vision, such as driving. In addition, expansion of the UFOV through training could improve the level of visually functioning ability for both young and old adults.
In order to attract a person's visual attention, objects or visual stimuli must be salient or conspicuous against their background. The UFOV provides a measure of how well peripheral targets can alert the attentional system (i.e., draw a person's attention) to relevant information or events in a person's environment. There is a tradeoff between the duration of stimulus presentation and the size of the area in which the visual system can be alerted. Individuals with a smaller UFOV are at a disadvantage in terms of their ability to quickly re-orient their attention (for example, to a vehicle approaching from the side) because they will not perceive its approach until it is much closer than it would have to be for an individual with a larger UFOV. Thus individuals with a smaller UFOV, in order to attend to the same visual area, would require more visual fixations (eye movements) and each fixation would need to be of a longer durational time period. It is as if their window of visibility was two to three times smaller than that of an individual without this type of problem.